Vertical MOSFET generally have superior power switching performance when compared to conventional bipolar devices. However, the on-state resistance of power MOSFET increases sharply as breakdown voltage increases. As a result, vertical MOSFET may be unusable in high voltage applications.
One solution for achieving lower on-state resistance while maintaining reasonable breakdown voltage is by utilizing “super junctions.” FIG. 1 schematically illustrates a conventional n-type vertical MOSFET 10 with super junctions. As shown in FIG. 1, the MOSFET 10 includes a drain electrode 12 coupled to an n-type drain 13 at a first end 10a, a source electrode 14 coupled to an n-type source 20, and a gate 16 spaced apart from the drain electrode 12 at a second end 10b, and a drift region 18 between the first and second ends 10a and 10b. The MOSFET 10 also includes a p-type well 21 proximate to the source 14 electrode and the gate 16, forming the body region of the field effect transistor.
The drift region 18 includes a p-type pillar 22 juxtaposed with an n-type pillar 24 forming a “super junction.” The p-type pillar 22 and the n-type pillar 24 are doped with select ion concentrations such that these two pillars at least approximately deplete each other laterally. As a result, the MOSFET 10 may have a high break down voltage between the source 14 and the drain 12. In operation, the n-type pillar 24 forms a conduction channel between the drain 12 and the source 14. Compared with conventional power MOSFET, the n-type pillar 24 may be doped with higher concentrations and thus may have a low on-state resistance. Even though MOSFET with super junctions have many performance advantages, the manufacturing of such devices can be costly and imprecise. Accordingly, certain improvements are needed for efficiently and cost effectively forming small dimension pillars in vertical MOSFET.